1. Field of the Invention
The present invention relates to a semiconductor switch device for an alternating current (AC) power source, and more particularly to a semiconductor switch device that may shut down an AC power source for a load, e.g., an electric motor.
2. Description of the Related Art
In an AC power output of a conventional electric motor equipped with a distributing disc and a frequency converter, the main switch must trip in a case that the output end has a short circuit or there is a malfunction in grounding, Currently, the main switch adopts fuseless switches, electromagnetic switches, or other breakers that operate mechanically and thus have a slow acting speed. Sparks are apt to occur and are thus dangerous and the longevity is relatively short. The slow acting speed of the switches tends to result in damage to the load when malfunctional current occurs in the load side. For example, a semiconductor type frequency converter does not allow over-current to exist more than 10 microseconds. However, it is impossible for current fuseless switches or breakers to reliably cut off the current within 10 microseconds. In addition, in an automated production systems (e.g. yarn coilers or conveying systems for steel-rolling apparatus), a frequency converter is in synchronous operation with tens of electric motors such that the whole production system stops once the frequency converter trips due to a short circuit of one of the electric motors or malfunction in grounding.
The switch circuits for soft starters, power factor controllers, and voltage-variable governors of traditional electric motors adopt reverse blocking triode thyristors wherein half of the waveforms is passed through a thyristor to delay activation for reducing the voltage. However, since the voltage waveforms are not sine waves, the power factor and the starting torque for the electric motor are low, the soft starting effect is thus poor. For example, the maximum starting current for an electric motor manufactured by Simens is 500% of the rated value, yet the maximum starting torque is only 25% which is only suitable for blowers and water pumps that have low starting torque. Application of the electric motor is thus severely limited such that the conventional soft starters cannot replace the starting devices of all kinds of electric motors.
A semiconductor switch device for an AC power source in accordance with the present invention comprises:
a plurality of first semiconductor elements and a plurality of second semiconductor elements connected in series with a plurality input alternating current (AC) sources of an AC power source, respectively, an end of each of the second semiconductor elements being electrically connected with each other;
a plurality of output AC sources each of which is formed between an associated said first semiconductor element and an associated said second semiconductor element, each said output AC source being electrically connected to a load;
a plurality of first reverse diodes each of which is connected in parallel with an associated said first semiconductor element; and
a plurality of second reverse diodes each of which is connected in parallel with an associated said second semiconductor element.
The AC power source can be cut off within 2 microseconds by means of supplying a control signal to the gates of the semiconductor elements. A continuous closed circuit is formed by the load and one of the second semiconductor elements when the AC power source is cut off, thereby avoiding momentary over-voltage. The AC source can be cut off at any time without the need to cut off after the voltage reaches zero, thereby providing a reliable protection immediately.
Each of the first semiconductor elements and the second semiconductor elements is an insulated gate bipolar transistor, metal-oxide semiconductor field-effect transistor, or giant transistor. Each of the first semiconductor elements and the second semiconductor elements includes a gate that equally divide a sine wave of the AC power source by a control signal of pulse width modulation, thereby modulating voltage to be outputted under cooperation with time control of pulse width modulation. The load may be a three-phase or single-phase electric motor.
The semiconductor switch device in accordance with the present invention, under cooperation with simple control functions, may replace conventional fuseless switches, electromagnetic switches, leakage breakers, thermal relays, and fuses. In addition, by means of cooperating with software and hardware, the semiconductor switch device in accordance with the present invention may acts as the soft starter, power factor controller, and voltage-variable governor of an electric motor, thereby providing precise and highly efficient operation.
In addition, electric arcs are not generated during operation of the semiconductor switch device in accordance with the present invention. The loss is small and the longevity is long. Only a voltage of 15 volts is sufficient to operate a current up to tens of thousands amperes, which may save the cost largely. Furthermore, by means of digital operation, operation of the semiconductor switch device in accordance with the present invention is simple, reliable, and highly efficient, and the semiconductor switch device can be communicated with a computer to proceed with centralized control.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.